These experiments focus on the mechanical properties and ATPase activities of single striated muscle cells in order to study the regulation and chemomechanical properties of actomyosin interactions in a 3-dimensional intact, force-generating system. The first hypothesis to be tested is that phosphorylation of the 18-20 k dalton light chains, the so-called P-light chains or regulatory light chains, provides a mechanism for modulating actomyosin kinetics and so a mechanism to regulate velocity of shortening and ATPase activity when the system is activated by Ca++. The first set of experiments is designed to test hypotheses generated from our recent work on the role that myosin light phosphorylation plays in mammalian muscle energetics and mechanics wherein the postulated regulation has an apparent physiological role. The second hypothesis is that changes in the substrate (MgATP) or product (ADP and Pi) concentrations and their ratios change actomyosin chemical and mechanical kinetics in ways that test crossbridge models. The second set of experiments thus explores the details of crossbridge kinetics and is designed to generate information that allows a connection to be made between the biochemical properties of contractile proteins and muscle physiology. Detailed kinetic mechanisms for any phosphorylation effects found will also be examined by these methods. Rabbit fast skeletal muscle fibers (psoas) and slow skeletal muscle fibers (soleus) will be used in order to determine the generality or specificity of the results obtained and to define qualitative and quantitative differences that may occur among these two fundamental types of skeletal muscle fibers.